diff options
Diffstat (limited to 'libc/src/__support/RPC/rpc.h')
-rw-r--r-- | libc/src/__support/RPC/rpc.h | 47 |
1 files changed, 4 insertions, 43 deletions
diff --git a/libc/src/__support/RPC/rpc.h b/libc/src/__support/RPC/rpc.h index fc7a66f4b88d..5f6c149b84ba 100644 --- a/libc/src/__support/RPC/rpc.h +++ b/libc/src/__support/RPC/rpc.h @@ -107,55 +107,16 @@ template <bool InvertInbox> struct Process { } /// Attempt to claim the lock at index. Return true on lock taken. - /// lane_mask is a bitmap of the threads in the warp that would hold the - /// single lock on success, e.g. the result of gpu::get_lane_mask() /// The lock is held when the zeroth bit of the uint32_t at lock[index] /// is set, and available when that bit is clear. Bits [1, 32) are zero. /// Or with one is a no-op when the lock is already held. - [[clang::convergent]] LIBC_INLINE bool try_lock(uint64_t lane_mask, - uint64_t index) { - // On amdgpu, test and set to lock[index] and a sync_lane would suffice - // On volta, need to handle differences between the threads running and - // the threads that were detected in the previous call to get_lane_mask() - // - // All threads in lane_mask try to claim the lock. At most one can succeed. - // There may be threads active which are not in lane mask which must not - // succeed in taking the lock, as otherwise it will leak. This is handled - // by making threads which are not in lane_mask or with 0, a no-op. - uint32_t id = gpu::get_lane_id(); - bool id_in_lane_mask = lane_mask & (1ul << id); - - // All threads in the warp call fetch_or. Possibly at the same time. - bool before = - lock[index].fetch_or(id_in_lane_mask, cpp::MemoryOrder::RELAXED); - uint64_t packed = gpu::ballot(lane_mask, before); - - // If every bit set in lane_mask is also set in packed, every single thread - // in the warp failed to get the lock. Ballot returns unset for threads not - // in the lane mask. - // - // Cases, per thread: - // mask==0 -> unspecified before, discarded by ballot -> 0 - // mask==1 and before==0 (success), set zero by ballot -> 0 - // mask==1 and before==1 (failure), set one by ballot -> 1 - // - // mask != packed implies at least one of the threads got the lock - // atomic semantics of fetch_or mean at most one of the threads for the lock - return lane_mask != packed; + LIBC_INLINE bool try_lock(uint64_t, uint64_t index) { + return lock[index].fetch_or(1, cpp::MemoryOrder::RELAXED) == 0; } // Unlock the lock at index. - [[clang::convergent]] LIBC_INLINE void unlock(uint64_t lane_mask, - uint64_t index) { - // Wait for other threads in the warp to finish using the lock - gpu::sync_lane(lane_mask); - - // Use exactly one thread to clear the bit at position 0 in lock[index] - // Must restrict to a single thread to avoid one thread dropping the lock, - // then an unrelated warp claiming the lock, then a second thread in this - // warp dropping the lock again. - uint32_t and_mask = ~(rpc::is_first_lane(lane_mask) ? 1 : 0); - lock[index].fetch_and(and_mask, cpp::MemoryOrder::RELAXED); + LIBC_INLINE void unlock(uint64_t, uint64_t index) { + lock[index].store(0, cpp::MemoryOrder::RELAXED); } }; |